Chapter: Civil Surveying : Levelling and Applications
important Questions and Answers: Levelling and Applications
LEVELLING
AND APPLICATIONS
1.
Define Levelling. What are the uses of leveling?
Levelling is a branch of
surveying, the object of which is; (i). To find the elevations of given points
with respect to a given or assumed datum, and (ii). To establish points at a
given elevation or at different elevations with respect to a given or assumed
datum.
2. Define benchmark and reduced level. Benchmark:
Benchmark is a relatively
permanent point of reference whose elevation with respect to some assumed datum
is known.
Reduced
level or Elevation:
The vertical distance of a point
above or below the datum is known as the elevation or R.L of that point. R.L of
a point may be positive or negative according as the point is above or below
the datum.
3.
What are the different kinds of bench marks?
A BM is the reference point of
known elevation. It may be classified into following types.
(i).G.T.S Bench Mark: The
great trigonometrical survey (G.T.S) bench marks are established by the survey
of India throughout the country. The levels of this bench marks are established
very accurately at a large interval with respect to the mean sea level at
Bombay port.
(ii). Permanent Bench Mark:
These are established by different Government departments like PWD, Railways,
Irrigation etc,. The RL of these points are determined with reference to the
G.T.S Bench Marks. Points on rocks, culvert, gate pillars, etc,.
(iii).Temporary Bench Mark:
These are established temporarily whenever required. These are generally chosen
to close the day's work and to start the next
days. Points on roofs, walls, basements, etc,.
(iv). Arbitrary Bench Mark:
When the RL of some fixed points are assumed, they are termed arbitrary
benchmark.
4.
What do you mean by datum surface?
It is any surface, to which
elevations are taken as a reference for the determination of elevations of
various points. In India the datum adopted for the great trigonometrical survey
(G.T.S) is the mean sea level at Bombay port.
5.
What is mean by line of collimation and height of collimation?
Line of collimation (Line of sight): It is an
imaginary line passing through the intersection of the cross hairs at the
diaphragm and the optical centre of the object glass and its continuation.
Height of collimation(HOC): The
elevation of the Line of collimation (Line of sight) is known as Height of
collimation. HOC = RL of BM + BS.
6.
Write the different types of levels.
1. Dumpy
level
2. Tilting
level
3. Quick
setting level.
4. Y-level.
5. Reversible
level.
6. Automatic
level
7. Lazer
level.
7. List the
essential parts of a Level.
Telescope,
Eyepiece, Objective
Focusing
screws. Longitudinal bubble
Foot
screws, Upper parallel plate ( Tribrach), Foot plate( Trivet).
Diaphragm
adjusting screws, Bubble adjusting screws, Tripod.
8. What are
the different type's leveling staffs?
1. Target
Staff
2. Self
reading Staff.
a.Solid Staff
b. Folding
Staff
c. Telescopic
Staff.
9.
Define and distinguish between 'Back sights' and 'Fore sight' in the
process of fly Levelling.
Back sights (BS)
1.This is the first staff reading
taken in any setup of the instrument. It is always taken on a point of known
elevation( BM).
2.It is used to determine the
height of the instrument.
HI = known RL + BS
Fore sight (FS)
1.This is the last staff reading
taken in any setup of the instrument and, after that instrument is shifted.
2.It is used to determine the
elevation (RL) of the staff station.
RL = HI - FS.
10.
Explain the theory of direct leveling.
It is the branch of leveling in
which the vertical distances with respect to a horizontal line may be used to
determine the relative difference in elevation between two adjacent points.
Steps involved:
HI = known RL(BM) + BS RL = HI - FS.
11.
Distinguish between differential levelling and reciprocal levelling.
Differential
levelling
1.Difference
in elevation between two or more points is determined by without
any regard to the
alignment of the points is called differential
leveling.
2.It
is used when:
(i). two points are a large distance
apart.
(ii). The difference in elevation
between two points is large.
(iii).
Some obstacles intervenes between
the points.
Reciprocal
levelling
1.Difference
in elevation between two points is accurately determined by two sets of reciprocal observations..
2.It
is used when:
(i). The instrument cannot be setup
between the two
points due to an
obstruction such as a valley,
river, etc,.
12.
Reduced level of Bench Mark A - 50.000m
Reading
on staff held at A - 2.435m
Reading
on staff held at station point B - 1.650m
Find: (a)
Height of collimation.
(b) Reduced level of station point B.
(c) Rise/fall of B with respect to A.
(a).
Height of collimation = RL of BM A + BS
|
(HOC) |
= 50.000 + 2.435 |
= 52.435m
(b)
Reduced level of station point B.
= HOC - FS.
= 52.435 -
1.650
= 50.785 m
(
c). Rise/fall of B with respect to A.
= 2.435-
1.65 ( Lower staff reading being higher)
= 0.785m,
= with
compare to A, the station point B being 0.785m higher.
13. Compare height of collimation
method and rise and fall method.
a. Height of
collimation method
b.Rise and fall method
1.
a.It is more rapid,
less tedious and It is as it involves few calculation. involving
b.more laborious and
tedious , simpler several calculations.
2.
There is no check on
the RL of the intermediate points.
There is a check on the
RL of the intermediate points.
3.
Errors in intermediate
RL's cannot be detected.
Errors in intermediate
RL's can be detected.
4.
There are
two arithmetic checks
on the accuracy of RL
calculation. ?BS -?FS = Last RL -First RL.
There are three
arithmetic checks on the accuracy of
RL calculation. ?BS -?FS = ?Rise -?Fall =Last RL -First RL.
5.
It is suitable in the
case of L.S and C.S, Contour etc.
It is suitable in fly
leveling where I intermediate sights
are less.
14. Write the
formula for curvature correction, refraction correction and combined
correction.
Curvature correction CC = 0.07849 d2
( negative) m Refraction correction Cr = 0.01121 d2 (positive) m
Combined correction. C = CC - Cr =
0.06728 d2 (negative) m.
Note: 'd' is to be
substituted in Km, while the corrections will be in m.
15.
List out the various sources of errors in
levelling.
Three
principal sources:
(i).
Instrumental error
a. Error due
to imperfect adjustment
b. Error due
to sluggish bubble.
c. Error due
to movement of objective slide.
d. Error due
to defective joint.
e. Error due
to incorrect length of staff.
(ii).
Natural error.
a. Earth's
curvature.
b. Atmospheric
refraction.
c. Variations
in temperature.
d. Settlement
of tripod.
e. Wind
vibrations.
(iii).
Personal errors.
a. Mistakes
in manipulation.
b. Mistake
in staff handling
c. Mistake
in reading the staff.
d. Error's in
sighting.
e.
Mistakes in recording.
16.
List out the leveling problems.
1. Levelling
on Steep Slope.
2.
Levelling on Summits and Hollows. 3.Taking Level
of an Overhead Point.
4. Levelling
Ponds and Lakes too Wide to be Sighted across.
5. Levelling across River.
6. Levelling
on Past High Wall.
17.
Define sensitivity of a bubble. State any two
factors affecting the same.
The sensitiveness of a bubble is
defined the angular value of one division of the bubble tube. It means the
capability of showing small angular movements of the tube vertically. It can be
increased by:
1. Increasing
the internal radius of the tube.
2. Increasing
the diameter of the tube.
3. Increasing
the length of the tube.
4. Decreasing
the roughness of the walls.
5. Decreasing
the viscosity of the liquid.
18.
What is a spire test?
It is used to make the horizontal
axis perpendicular to the vertical axis. This test is also known as the test
for the adjustments of the standards. It is done by means of the adjustments of
the vertical hair. It is one of the permanent adjustment of the level and
theodolite.
19.
Define Contour, contour interval and, horizontal equivalent.
Contour: A
contour is an imaginary line on the ground joining the points of equal
elevation.
Contour interval: It is
the vertical distance between any two consecutive contours. It depends upon the
nature of the ground, the scale of the map and the purpose of the survey.
Horizontal equivalent: It is
the horizontal distance between any two consecutive contours. It varies
according to the steepness of the ground.
20.
What are the different Characteristics of contour?
1. Contour lines are closed
curves. They may either within the map itself or outside the map depending upon
the topography.
2. Uniformly
spaced, contour lines indicate a uniform slope.
3. A series
of closed contours with increase in elevation from outside to
inside
in plan denotes a hill.
4. A series of closed contours
with increase in elevation from inside to outside in plan denotes a depression.
5. The spacing between the
contour lines depends upon the slope of the ground. In steep slopes, the
spacing is small and for gentle slope, the spacing is large.
21.
What are the uses of contours?
1. Volume of
earthwork for any work can be estimated.
2.
The
capacity of the
reservoir or the
area of the
catchments can be
calculated.
3. Very useful in military
operations to decide the position of the guns, the line of march.
4. Longitudinal and cross section
can be drawn along any direction to know the nature of the ground.
22.
Write the different formulae to calculate the area
of the irregular plate.
1. By
computations based directly on the field measurements:
(i). By dividing the area into number of
triangles. (ii). By offsets to base line.
a.Mid ordinate rule = ?O .d
b. Average
ordinate Rule = nd. ?O
n+1
c. Trapezoidal rule = ( Oo +
On + Ol +O2+O3+
On-1)
d 2
d. Simpson's rule =
[( Oo + On )+ 4( O1+ O3+
. On-1)
+ 2( O2+ O4+
. On-2
) ] (d/3)
Oo = Ordinate at one end.
On = Ordinate at other end.
Ol +O2+O3+
On-1
= Ordinate at end of each divison. d = Length of the base
(iii).
By latitudes and Departures:
a.By double meridian distance
(D.M.D. method).
b.
By double parallel distance (D.P.D. method). (iv).
By co- ordinates.
2. By
computations based on measurements scaled from a map.
3. By
mechanical method: By means of planimeter.
23.
How do you calculate the capacity of the reservoir from the contour map?
From
the contour map, the capacity of the reservoir is calculated by the
following
formulas.
1. Trapezoidal rule = ( Ao +
An + Al +A2+A3+
An-1)
2
2.
Prismoidal rule = [( Ao + An )+ 4( A1+
A3+
. An-1) +
2(
A2+ A4+
.
An-2
) ] (d/3)
Ao, A1, A2,
A3,
An = Areas enclosed by successive
contours. d = contour interval.
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